The method and apparatus disclosed herein is for producing digitally subtracted real-time or continuous images, serial images and integrated images for video recording and display to enable dynamic non-invasive x-ray studies of rapidly moving organs such as the heart and, in particular, the blood vessels of such organs. The aforesaid technique is frequently referred to as digital subtraction fluoroscopy or radiography.
Prior examples of apparatus for performing real-time digital subtraction fluoroscopy are disclosed in U.S. Pat. Nos. 4,204,225 and 4,204,226, both issued on May 20, 1980.
One object of digital subtraction fluoroscopy is to obtain a visible representation of an x-ray image in which the anatomy of interest, usually the blood vessels, is emphasized and the anatomy which simply contributes to background and obscures the vessels is deemphasized. This object is achieved in the cited patents by converting successive x-ray images obtained with constant x-ray spectra to optical images, converting the optical images to analog video signals with a television camera, converting the analog signals to digital words which compose a frame and represent the individual picture elements comprising the images, storing the data for an image field or frame and then subtracting the data for the current frame from the preceding and/or succeeding frame alternately and repeatedly to yield data for displaying the results of subtraction as a succession of subtracted images on a television screen. The data for one or both or even more images in succession are usually weighted or variously operated on to bring about cancellation of obscuring background and to have the anatomy of interest remain.
An x-ray opaque dye, such as an iodine compound in solution, is usually injected intravenously remotely from the heart to aid in delineating the blood vessels when the dye reaches the heart and to afford an opportunity for determining how well the dyed blood circulates through the vessels.
Since the heart is in motion, registration of successive images which are subject to subtraction becomes problematical and some blurring of the displayed image results. The problem can be more difficult to deal with where, as in the prior patents, each x-ray view is made with the x-ray tube operating at the same anode-to-cathode voltage in which case the energy spectrum of the x-ray beam is fixed. This results in the intensities of the successive images being substantially the same, except for such differences as may result from the dye-carrying blood advancing further from view to view.
In prior art methods, several successive frames are integrated to reduce the effects of x-ray statistical noise and electronic system noise. The heart can move during the integrating interval and blurring is more likely to be noticeable in the displayed image. Moreover, if time must always be allowed for integrating, continuous and truly real-time images cannot be obtained. At best, perhaps fifteen subtracted frames per second could be displayed which is well below the thirty or sixty frames per second which should be obtainable in a 60 Hz synchronized television system.
In the successively integrated or time interval differencing mode of operation described in the prior patents, the maximum contrast obtainable in an image is that due to the difference in contrast between successive images and, as has been indicated, this mode only images what is changing between successive TV fields or frames. The change may be due to motion of the heart, for example, superimposed on a background consisting of unmoving tissue or bone in which case the moving organ must necessarily be a little blurred because of the time lapse between successive frames. In cases where an x-ray opaque contrast medium is being observed as it flows through the blood vessels, the only contrast difference which occurs between frames is that which is due to the leading edge of the opaque medium having advanced during the integrating interval between frames.